Constant selectivity receiver



Feb. 6, 1940. R. A. BRADEN 2,189,308

CONSTANT SELECTIVITY RECEIVER Original Filed Jan. 6, 1951 H w ATTORNEY hul PatentedFeb. 6, 1940 UNITED STATES ,.PATEENT., foFFIcI-:jj

i 2,189,308 coNsTANr sELEcriviry RECEIVER Rene A. BrademMerchantville, N. assigner to Radio Corporation of America, a corporation of Delaware Application' January 6, 1931,` Serial No.` 506,877 i .Renewed February 1,1933 j 5 claims. (o1. 25o- 20)` present invention relates to radio receivers, and more particularly to a radio receiver employinga compensated audio frequency amplifier. f It is Well known in the prior art that a radio frequencyr amplifier employing tuned circuits tendslto attenuate the side' band frequencies more or less seriouslypaccording to the sharpness of tuning, the number of stages of amplication,

the wave length and other well known factors.' Thus, a '1,000 cycle side band will be attenuated to a certain percentage of the carrier amplitude, a

2,000 `cycle side band will be cut to a smaller percentage, while a` 5,000y cycle side band will be attenuated to a stillsmaller percentage of `the cartier amplitude,` and vso on.

Again, those skilled in the receiver art, well known that in any commercial tuned radioffrc4 quency amplifier the amount of attenuation of the side band of `frequencies depends on the wave length, becoming more serious as the wave length increase'. In other words, in broadcast receivers y employing a single tuned circuit in each stage, it

kisl a common characteristic of such receivers to i tune too sharply at'low 'carrier frequencies, and too broadly at `high carrier frequencies;

A careful analysis of the problem has shown that there are two` important phases to be considered. In therst place, it is essential to design the radio frequency `amplifier insuch a manner that the attenuation of the side band frequencies transmitted through the `amplifier will be the saine at all wave lengths. In the second placer, it is equally important to design the audio ,y

frequency amplier which includes at leastfonetuned circuit in each pair of `coupled input and `output circuits, the tuned circuit possessing a 'constant resistance over apredeterrnined range of frequencies whereby the sharpness of tuning is Vsubstantially the same at all frequencies Within a predetermined range, such as the broadcast range.,. In `this way, aA tuned radio `frequency j amplifier is provided which does not tune too sharply at low carrier frequencies, and too-broadly at high carrier frequencies, but rather has a constant selectivity characteristic throughout the desired radio ireque'rrcy range.

Now, I have discovered a "novel method of,`

and devised means for, solving the" second phase of `the 'aforementioned problem, the methodutil- "izing 'the principles disclosed in my aforemen-A tioned copending app1ication`and an audio fre-` quency amplier having a characteristic which essentially permits increase of amplification with increase of frequency upto *theV greatest `value of audio frequencydesiredtobe` amplified, as for example 5,000 cycles, whereby `the audio frequency'ampliiication of the receiver willbe ap- 4panfently the same at all carrier frequencies.

Accordingly, it is one of the main objects of my present invention to provide `a radioreceiver comprising a constantlselectivity "tuned radio frequency amplifier combined with an audio frequency amplifiedwhose fidelity curve is modified so that the overall curve of the receiver is practically flat up tol 5,000 cycles whereby a receiver of good fidelity is obtained.`

Another4 importantfobject of the present in vention is to provide a method off, andmeans for,

operating a radio receiverconsisting in kmain-,-

taining the radio frequency resonance curve la` constant shape at all wave lengths, and simul-l` taneouslyV employing an audio frequency amplifier whose frequency' characteristic is the inverse of the radio 4frequency resonance curve. I

`Anotlierobjec'zt of the invention is to providea radio broadcast receiver arrangement utilizing a compensating audio frequency amplierwith a constant selectivity radio 'frequencyamplifier for bringing up thehi'gh notes tothejsame amplitude as the low notes whereby improved quality of re`- ception is secured throughout the broadcast range without impairng the` selectivity'of reception, in

any way. i I y 1, Other objects ofthe invention are to improve generally the simplicity and efficiency of radio receivers, andto particularly provide a receiver utilizing ,a constant selectivityf'radio frequency amplication in ,combination with a compensating audio frequencyamplifier `which is reliable in operation, possesses a flat yover-all characteristic upto 5,000 cycles, and is, in addition, `economical tol construct.

in Y

The novel features which Irjbelieve Vto be charv acteristic of my invention are `set forthfin par-` ticularity inthe appendedclairns, the invention."

itself, however, as to both `its organizationand methodofoperation will best be understood by" reference to fthe following description taken in connection with the drawing' in which I have in` `dicated dagrammaticallyone `circuit organiza-V tion whereby my invention may be carried into effect.

In the drawing:

Fig. 1 diagrammatically shows a receiver embodying the invention,

Fig. 2 is a graphic representation of the operation of the compensating audio frequency amplifier. y'

Referring to the accompanying drawing, there is shown in Fig. 1, in a diagrammatic manner, a radio receiver comprising the usual signal energyy collecting means consisting of an antenna A,

grounded as at G. The grounded antenna circuit is coupled, as at M, to the input electrodes of the screen grid tube I in the first stage 'of radio frequency amplification. The coupling .means M `the cathode in the well known manner.

comprisesv a primary coil y2- connected in the antenna circuit and a secondary coil, 3 connected between the control electrode and cathode of the tube I, the latter although being shown of the screenV grid type being adapted for substitution by any other type of Space discharge device, such asa triode. l

. The anode of the tube I is `connected to the positive terminal of a source of uni-directional current B through the primary coil 2', the negative'terminal Aof the source B being connected to The inputcircuit ofthe tube I is tuned by a Variable condenser 4, the latter being connected in series with a circuit generally denoted by the reference numeral 5. The circuit 5 comprises an inductance Lo in serieswith a resistance RL, the latter two elements being in shunt with the series connection between the capacity Co and resistance The primary coil 2' is coupled t0 the secondary coil 3', both coils constituting the coupling means 40 between the output circuit of tube I and the input circuit of screen grid tube vI5. In a manner similar to that shown in jconnection with the input circuit ofthe tube I, the input circuit of tube 6 is tuned by a variable condenser 4 connected in series with an impedance circuit 5, the

latter comprising the same elements as explained in connection with the impedance circuit in the input circuit of tube I. The anode of tube 6 is energized from a source B, the latter, if desired, being a source common with the source B connectedwith the anode of tube I.

The output circuit of the tube I5 is coupled, by the same type of coupling used in connection with the rst two tubes, to the input circuit of the screen grid tube 1, the control electrode circuit of this tube being arranged for detection. This is accomplished by the well known series capacity and shunt grid leak connection, generally denoted by the reference numeral 8, in the control electrode circuit, Here, again, a variable condenser pending application Serial No. 396,956, already referred to, the utilization of the impedance circuit 5 in series with the tuning condenser 4 in 'each tuned radio frequency stage results in a substantially vconstant degree of sharpness of tuning throughout a predetermined range of i'requencies, such as the broadcast range. Briey, it being understood that details of design are clearly disclosed in my aforementioned copending application, the tuned radio frequencyamplifler is rendered constantly selective by suitable selection of the values of the constants of the elements Lo, RL, Co and Rait being clearly understoodthat the present disclosure of the means for imparting a constant resistance characteristic to the tuned radio frequency circuits is not to be interpreted as limiting in any manner the mode of securing a constant selectivity characteristic. In fact, my aforementioned copending application discloses other modes and principles of securing the same result, the essential feature for the design of the tuned radio frequency stages being the production of amplifying tuned circuits possessing a constant effective resistance throughout a given frequency range.

Assuming, then, that the detector stage impresses across the primary coil I of the first audio'. transformer coupling Il, a band of audio frequencies having a width of substantially 10 kilocyclesthat is to say each side band is ,5,000 cycles wide, the remaining problem, solved by me in the present application, is to transmit this selected band of audio frequencies through the subsequent audio frequency amplifier in such a manner that the audio frequency output will be apparently the same at all audio frequencies within selected 10 kilocycle bands. The solution of this phase of the problem is now to be described in detail.v Across the primary coil IIJ is connected a series'connection of a resistor I2 and an inductance I3, it being understood that the anode of the detector tube I is maintained at the proper triode type being adapted to be substituted forv by any other desired type of space discharge device. The control electrode is preferably maintained at a proper negative biasing potential by connecting the low potential terminal of coil I4 to the negative terminal of a biasing source C, the positive terminal of the source C being connected to the cathode, in a manner well known to those skilled in the art, but not shown herein in order to preserve simplicity of description.

The tube l has its anode positively biased from a source-B2, it being obvious that this source mayT be the same source as the source B1, the anode being connected to the positive terminal of source B2 through the primary coil ID of the second audio transformer I l'. A series impedance path, composed of a capacity I6, a resistance I1, and an inductance I8, all in series, is connected across the primary coil I0'. The secondaryv coil I4 of transformer II is connected between the input electrodes of tube I9, the latter preferably being an audio power tube of any desired type.

The control electrode of tube I9 is negatively biased from a source C1,` in `a manner similar to the control electrode of tube I5, while the anode of tube I9 ispositively biased from a source B3 through the primary coil 20 of the output audio transformer 2|. It is of course understood that the sources B1, B2 and B3 can be the same source. y

and if desired, can even be the same source as the sources B in the radio frequency stages; and, in the same manner, the sources C and C1 can be a common source of biasing potential. The secondary coil 22 of output transformer ZI maybe connected to any well known type of utilization means, such as head phones, a loud speaker and the like.

In Fig. 2 there is shown, in a graphical manner, a curve which demonstrates the operation of the compensating audio frequency amplifier. It will be noted from the curve that the amplincation increases with frequency up to about 5,000 cycles in substantially direct proportion to increase in frequency from a point approximately at '700 cycles. In other words, an audio frequency amplifier is employed in the receiver circuit shown in Fig. l, whose frequency characteristic is the U quency amplifier.

inverse of the radio frequency resonance curve. rI'his characteristic is imparted to the` audio fre quency amplifierby the design, as shown in the audio stages in Fig. 1, of the primary coil circuits of the audio transformer Il and il. This may be explained in the following manner:

At low audio frequencies resistance i2 shunts current away from transformer coil i8, thus reducing amplification. At higher audio frequencies, coil i3 becomes effective, and the cir-` cuit l2, I3 has high enough impedance so that the amplification is only slightly reduced. The second audio transformer circuit acts in the same way, except that the greatest reduction of amplification occurs at the resonant frequency of I5, it.

(amount of reduction at this frequency being determined by Il).

Thus, Fig. 2 graphically shows the combined effect of both shunting circuits in the audiofre It is pointed out that while compensation in the audio frequency circuit could be applied to any receiver,the best results are secured when the amount of compensation required is the same at all carrier frequencies. This, of course, explains why a radio frequency circuit having constant selectivityl is employed.

While I have indicated and described one yarrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is byno means limited to the particular organization shown and described, but

modulation, said tuned circuit including an imi pedance element having resistance therein arranged in such a manner that the resonance curve ofthe coupledresonant circuits is substantially the same at whatever carrier frequencies in a desired carrier frequency range the said one circuit is tuned to, and a compensating audio frequency amplifier comprising at least two coupled stages, the coupling between `said stages being designed in such a manner that the ampliication of the audio frequency amplifier increases with audio frequency whereby the apparent audio frequency amplification of the side bands of selected carrier frequencies throughout said range is substantially uniform.

2. In combination, in a radio receiver, a radicfrequency amplifier, means for tuning said amplifier over adesired range of modulated carrier frequencies, yadditional means electrically associated with said tuning means and amplifier for producing a uniform selectivity characteristic for said amplifier, means for detecting selected modulated carrier energy, and an audio frequency ampliiier having an amplification characteristic which is directly proportional to audio frequencies to be amplified.

3. In combination, in a radio receiver, a selec-` detector, and an audio frequency amplifier including means to preserve an audio frequency amplification ratio between the detected selected carrier and side bands which is the inverse of said `radio frequency amplification ratio.

4. The method of receiving bands of radio fre- `quencies, corresponding to modulated carrier energy at different carrier frequencies, throughout a desired frequency range, which consists in selecting .a desiredband of radio frequencies comprising a carrier and its side bands due to modulation, subjecting the selected band to radio frequency amplification, selectively amplifying the amplified band at least another time whereby band varies inversely according to a predetermined` relation with the spacing between the said given frequency and the carrier, detecting the selectively amplified band of frequencies to produce a bandof audio frequencies corresponding to the selected bandof radio frequencies, amplifying the band of audio frequencies in a manner inverse to that` at radio frequency amplication, and maintaining said predetermined radio frequency amplification relation substantially constant for all selected bands of radio frequencies throughout said desired frequency range.

5. An apparatus for receiving a carrier wave modulated by a desired signal including, in combination, selective receiving means, the selectivity of said means being such that the amplitude of the modulation frequencies over a substantial portion of the modulation frequency range will be relatively altered, means for relatively altering the amplitude of the said modulation frequencies inthe opposite sense, and additional means, electrically' associated with said selective receiving means, for maintaining the said first alteration substantially constant over the tuning range of said receiving means.

RENE A. BRADEN.

' amplification of a given frequency in the selected 

